The present invention relates to the use of organic chemicals as tracers for monitoring subterranean fluids. As used in the specification and claims, the term subterranean fluids means aqueous-based fluids (e.g., ground water, geothermal brines, and fluids injected during the course of enhanced oil recovery procedures such as steam floods, carbon dioxide floods, caustic floods, polymer floods, and micellar-polymer floods), and organic-based fluids (e.g., crude oil) produced from a subterranean formation or subterranean reservoir.
In most production reservoirs, the produced brines are injected into the formation for purposes of maintaining reservoir pressure and avoiding subsidence and environmental pollution. In the case of geothermal fields, the brines are also injected to recharge the formation. However, the injected brines can adversely affect the fluids produced from the reservoir. For example, in geothermal fields, the injected brine can lower the temperature of the produced fluids by mixing with the hotter formation fluids. In order to mitigate this problem, the subsurface paths of the injected fluids must be known.
Tracers have been used to label fluids in order to track fluid movement and monitor chemical changes of the injected fluid. Despite their importance to the reservoir operator, very few tracers are presently available. Furthermore, of those that are available, little is known about their stabilities or behavior at the elevated temperatures that typify geothermal resources capable of electric power generation.
Radioactive materials are one class of commonly used tracers. These tracers have several drawbacks. One drawback is that they require special handling because of the danger posed to personnel and the environment. Another drawback is the alteration by the radioactive materials of the natural isotope ratio indigenous to the reservoir--thereby interfering with scientific analysis of the reservoir fluid characteristics. In addition, the half life of radioactive tracers tends to be either too long or too short for practical use.